hasMany Relation Count Accessor not triggering

I have a parent - student relationship and I want to return a count of students.

The relationship works fine and I get back the correct count. However the accessor does not trigger and so I simply get an array like a normal relation. I use the Laravel Debug Bar and have tried to log some text from the accessor but the line of code seems to never execute.

I followed Jarek's blog post to get this far: http://softonsofa.com/tweaking-eloquent-relations-how-to-get-hasmany-relation-count-efficiently/

Here is my code:

//Relation
public function studentCount()
{
    return $this->hasOne('App\Models\ParentLink', 'parent_id')->selectRaw('parent_id, count(*) as count')->groupBy('parent_id');
}

//Accessor
public function getStudentCountAttribute()
{
    Debugbar::info("hey");

    //if relation is not loaded already, let's do it first
    if ( ! array_key_exists('studentCount', $this->relations)) 
    {
        $this->load('studentCount');
    }
    $related = $this->getRelation('studentCount');
        
    // then return the count directly
    return ($related) ? (int) $related->count : 0;
}

Returns only:

"student_count":{"parent_id":"2","count":"4"}